The strontium-isotope evolution of the Earth’s silicate sediments indicates that they must interact with a strontium reservoir whose Sr-87/Sr-86 ratio is relatively low. The Sr-isotope budget for these sediments can be balanced if the Sr lost to solution by weathering is replaced by Sr from the oceans (through diagenesis) and from carbonate sediments (through metamorphic decarbonation reactions). Because of the latter connection, the Sr-87/Sr-86 ratio of silicate sediments can constrain the contribution of metamorphism to the Earth-atmosphere CO2 flux.